The outer limits of the system of planets around our own Sun, where the influence of our local star ends, are being probed by the Voyager spacecraft, which were launched in 1977 on a five year mission to study Jupiter and Saturn.

The two nuclear powered probes continued to speed onwards to the outer Solar System, each flying in slightly different directions, with Voyager 1 becoming the most distant man-made object in space in the 1990s.

Today, in Nature, an analysis of recent data streamed back from the Voyager 2 spacecraft helps build up a picture of how the Sun interacts with the rest of the galaxy. The current mission of both spacecraft is to reach and study the outer limits of the heliosphere - a magnetic 'bubble' around the Solar System created when the particles that stream out from the Sun crash into and hold back the soup of particles in the rest of interstellar space.

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When the solar wind senses the edge of the bubble, called the heliopause, located at 7-8.5 billion miles from the Sun, it prepares for the impending collision at the "termination shock", where the solar wind slows down to subsonic speed Prof Edward Stone of Caltech and colleagues report that Voyager 2 crossed this boundary closer to the Sun than expected, suggesting that the heliosphere in the south is dented, or pushed in, closer to the Sun by the interstellar magnetic field.

Voyager 1 passed the termination shock at about 8.7 billion miles from the Sun, while Voyager 2 reached its more southerly edge, sooner than expected, passing the shock at about 7.8 billion miles. This reveals that the heliosphere is squashed inward in the south compared to the north.

Dr Rob Decker of the Johns Hopkins University, one of the team studying Voyager 2 measurements, said: "The squashing is rather small (a 10 percent effect at best), and competing modelling teams are still arguing over details." Dr John Richardson of the Massachusetts Institute of Technology, Cambridge, who discusses what happens to the energy of the solar wind in another Nature paper, added that the little probes will begin true interstellar travel in another decade.

"We hope the Voyagers will cross the heliopause boundary in about 10 years and be the first spacecraft to measure what is outside of the Sun's heliosphere."

Voyager 2 was launched Aug. 20, 1977. It visited four planets and their moons in the course of its journey into space. Its sister spacecraft Voyager 1, which was launched Sept. 5, 1977, crossed the termination shock in December 2004 - earlier than Voyager 2 because of a faster trajectory.

Operating in remote, cold and dark conditions, and powered by long-life nuclear batteries in the absence of solar energy, the probes are still transmitting data back to Earth, long after their original missions ended.

In other work, Nasa's STEREO spacecraft unexpectedly detected particles from the edge of the solar system last year, allowing University of California, Berkeley, scientists to map the region where the hot solar wind slams into the cold interstellar medium.